1. Technical Field
The invention refers to a device for transmitting signals between two terminals, at least one of which is moveable with respect to the other, and which are connected by at least one conducting line wound in turns. Other conducting lines can be connected between the two terminals. The line is stored in an essentially circular cassette comprising a stator that supports one of the terminals and a rotor that rotates around its axis and with respect to the stator and supports the other terminal. The turns of the line are subdivided into an inner winding area and an outer winding area with different winding directions, between which an approximately U-shaped reversing area is located. A ring-shaped guide encompasses the reversing area of the turns is located between the two winding areas and can move in the direction of the cassette. At least the inner periphery of the ring-shaped guide is configured to receive the turns of the inner winding area.
Such a device is needed, for example, to transmit a signal for releasing the impact-protection "airbag" in motor vehicles. It is located in the steering wheel of a motor vehicle to transmit an electrical or an optical signal. In the sense of the invention, the "line" can therefore be an electrical or an optical fiber line.
2. Description of the Prior Art
A significant problem for this device is the transmission of signals between fixed and moveable parts of a motor vehicle. The sliding contacts or rings, which have been known and used for a long time for transmitting current in such cases, are subject to wear and are disadvantageous, particularly because of the fluctuating transition resistances occurring at low power.
With the known device according to EP-OS 0 417 350, electric current is used to transmit the signals. The transmission of current takes place through a flat ribbon line or cable which is wound into a coil by a type of spring barrel and is stored in a cassette. With a relative rotating movement of the two terminals connected by the flat ribbon line, the coiled flat ribbon line "breathes" like the spring of a watch. In one direction of rotation, the turns of the coiled flat ribbon line are drawn together into a smaller diameter. They return to a larger diameter in the other direction. In this way the flat ribbon line is able to follow the turns of a steering wheel without any interruption of the signal path. To be able to perform a sufficient number of turns in both directions, a correspondingly long flat ribbon line is needed, which is stored in a large number of turns in the cassette. In principle, this device can be used in practice. However, because of the long length of the line and its relatively high ohmic resistance, difficulties can occur with the signals to be transmitted, especially at low power.
In the device according to the DE-OS 42 11 264, the length of the line between the two terminals is significantly shorter than in the device described in EP-OS 0 417 350. The coiled line is subdivided into inner and outer winding areas, in which the turns are wound in different directions. The two winding areas are interconnected by an approximately U-shaped reversing area. A ring-shaped guide is located between the two winding areas. It encompasses the reversing area and can move in the peripheral direction of the cassette. It is moved by the coiled line or by its reversing area when the rotor of the cassette is rotated. The guide comprises several rollers, which are offset from each other in the circumferential direction and can rotate around an axis, which are contacted by one turn each of the line from the inner winding area and the line from the outer winding area when the rotor is rotated. The rollers prevent the inner winding area turns from being applied too hard against the guide, which would jam it. The guide in this known device is especially expensive because of the separately located rollers.